The present invention is directed to gas discharge lamps. More specifically, the present invention is directed to a control circuit for operating a gas discharge lamp and measuring the average power delivered to the lamp.
Gas discharge lamps are used in a variety of applications. For example, mercury vapor lamps are used for ultraviolet (UV) curing of ink in printing presses, for curing furniture varnish, in germicide equipment for killing germs in food and its packaging and for killing bacteria in medical operating rooms. Many other applications also exist.
A traditional circuit for controlling a mercury vapor lamp includes an AC power source which drives a primary side of a ballast transformer. A secondary side of the transformer is coupled to the lamp. The lamp includes a gas-filled tube with electrodes at each end of the tube. The secondary side of the transformer applies a voltage between the electrodes which accelerates electrons in the tube from one electrode toward the other. The electrons collide with gas atoms to produce positive ions and additional electrons. Since the current applied to the gas discharge lamp is alternating, the electrodes reverse polarity each half cycle.
Since the collisions between the electrons and the gas atoms generate additional electrons, an increase in the arc current causes the impedance of the lamp to decrease. This characteristic is known as xe2x80x9cnegative resistance.xe2x80x9d The lamp is unstable, and current between the electrodes must be limited to avoid damaging the lamp. As a result, a typical control circuit includes a current limiting inductance coupled in series with the lamp. The inductance can either be a physically separate inductor or xe2x80x9cbuilt-inxe2x80x9d to the transformer as a leakage inductance.
When the lamp is first started, the lamp requires a very large striking voltage to initiate an arc to ionize the gas in the lamp. The electrodes of the lamp are cold and there are almost no free electrons in the tube. The impedance of the lamp is therefore very high. The voltage required to initiate the arc exceeds that required to sustain the arc. For example, the ignition voltage may be 1,000 volts while the operating voltage may be 550 volts.
One circuit for operating a gas discharge lamp and controlling its intensity is disclosed in U.S. Pat. No. 5,578,908, which is assigned to Nicollet Technologies Corporation of Minneapolis, Minn. This circuit uses a pair of anti-parallel silicon-controlled rectifiers (SCR""s) in series with the primary side of the ballast transformer for controlling the average AC power delivered to the primary winding and thus to the gas discharge lamp.
In most gas discharge lamp applications, there is a desire to control the light output accurately. The actual light output is proportional to the average power dissipated through the lamp. The average power dissipated through the lamp is the instantaneous product of the lamp voltage and lamp current averaged over one or more cycles. However, most traditional lamp control circuits control intensity by measuring either the lamp voltage or the lamp current which, by itself, does not give an accurate representation of the actual light output.
If the average lamp power was known, this value could be used to more accurately control curing times in UV curing processes and sterilization times in germicide equipment. One known method of obtaining the average lamp power is to use expensive test equipment, such as a digital oscilloscope. However, such test equipment is expensive, labor intensive and requires specialized knowledge to obtain and interpret its output. Alternatively, commercially available integrated circuits are available which could be used to digitize the lamp voltage and lamp current, multiply the digital values and average the results over time. However, these integrated circuits are also very expensive, and would therefore significantly increase the cost of the lamp control circuit.
Improved lamp control circuits are therefore desired, which have the ability to measure the average lamp power with relatively little added cost to the overall circuit.
One embodiment of the present invention is directed to a lamp power measurement circuit, which measures average power delivered to a gas discharge lamp. The circuit includes a voltage sensor having a first measurement output representative of AC voltage across the lamp and a current sensor having a second measurement output representative of AC current through the lamp. A first absolute value circuit is coupled in series with the first measurement output and has a first absolute value output. A second absolute value circuit is coupled in series with the second measurement output and has a second absolute value output. A pulse width modulator modulates one of the first and second absolute value outputs with the other of the first and second absolute value outputs and has a pulse width modulated output. A low-pass filter is coupled in series with the pulse width modulated output and has a DC voltage output representative of average power dissipated through the lamp.
Another embodiment of the present invention is directed to a gas discharge lamp control circuit, which includes alternating-current (AC) input terminals, lamp output terminals for coupling across a gas discharge lamp, and a ballast coupled between the AC input terminals and the lamp output terminals. A voltage sensor is coupled in the circuit to produce a first measurement output representative of AC voltage across the lamp output terminals. A current sensor is coupled in the circuit to produce a second measurement output representative of AC current through the lamp output terminals. A first absolute value circuit is coupled in series with the first measurement output and has a first absolute value output. A second absolute value circuit is coupled in series with the second measurement output and has a second absolute value output. A pulse width modulator modulates one of the first and second absolute value outputs with the other of the first and second absolute value outputs and has a pulse width modulated output. A low-pass filter is coupled in series with the pulse width modulated output and has a DC voltage output representative of average power dissipated through the lamp.
Another embodiment of the present invention is directed to a method of measuring power delivered to a gas discharge lamp by a lamp control circuit. The method includes: sensing a voltage representative of AC voltage delivered to the lamp and producing a first measurement output; sensing a current representative of AC current delivered to the lamp and producing a second measurement output; taking the absolute values of the first and second measurement outputs; pulse-width modulating one of the absolute values of the first and second measurement outputs with the other of the absolute values of the first and second measurement outputs to produce a pulse-width modulated output; and low-pass filtering the pulse-width modulated output to produce a DC voltage representative of average power delivered to the lamp.
Yet another embodiment of the present invention is directed to a gas discharge lamp control circuit, which includes alternating-current (AC) input terminals, lamp output terminals for coupling across a gas discharge lamp, and a ballast coupled between the AC input terminals and the lamp output terminals. Further, a voltage sensor senses a voltage in the circuit that is representative of AC voltage delivered to the lamp output terminals and produces a first measurement output. A current sensor senses a current in the circuit that is representative of AC current delivered to the lamp output terminals and produces a second measurement output. An absolute value circuit takes the absolute values of the first and second measurement outputs. A modulator pulse-width modulates one of the absolute values of the first and second measurement outputs with the other of the absolute values of the first and second measurement outputs to produce a pulse-width modulated output. A low-pass filter filters the pulse-width modulated output to produce a DC voltage representative of average power delivered to the lamp.